Potassium Nitrate (KNO3)

What is potassium nitrate?

The potassium nitrate is a metal alkali, and nitrate oxoanion potassium ternary compound salt. Its chemical formula is KNO 3 , which means that for every K + ion , there is a NO 3 ion interacting with it. Therefore, it is an ionic salt and constitutes one of the alkali nitrate (LiNO 3 , NaNO 3 , RBNO 3 …).

KNO 3 is a strong oxidizing agent due to the presence of the nitrate anion. That is, it functions as a reserve for solid and anhydrous nitrate ions, unlike other highly water-soluble or highly hygroscopic salts. Many of the properties and uses of this compound are due to the nitrate anion, rather than the potassium cation.

Potassium nitrate crystals

In the image above, some KNO 3 crystals are illustrated in the shape of needles. The natural source of KNO 3 is saltpeter, known by the names Saltpeter or salpetre , in English. This element is also known as potash nitrate or nitro mineral.

It is found in arid or desert areas, as well as efflorescence from cavernous walls. Another important source of KNO 3 is guano, the excrement of animals that inhabit dry environments.

Chemical structure of potassium nitrate

Crystal structure of potassium nitrate

In the upper image the crystal structure of KNO 3 is represented . The purple spheres correspond to the K + ions , while the red and blue spheres are the oxygen and nitrogen atoms, respectively. The crystalline structure is of the orthorhombic type at room temperature.

The geometry of the anion NO 3 is that of a trigonal plane, with the oxygen atoms at the vertices of the triangle, and the nitrogen atom at its center. It has a positive formal charge on the nitrogen atom, and two negative formal charges on two oxygen atoms (1-2 = (-1)).

These two negative charges of NO 3 delocalize between the three oxygen atoms, always maintaining the positive charge on nitrogen. As a consequence of the above, the K + ions of the crystal avoid being located just above or below the nitrogen of the NO 3 anions .

In fact, the image demonstrates how the K + ions are surrounded by oxygen atoms, the red spheres. In conclusion, these interactions are responsible for the crystal arrangements.

Other crystalline phases

Variables such as pressure and temperature can modify these arrangements and originate different structural phases for KNO 3 (phases I, II and III). For example, phase II is that of the image, while phase I (with trigonal crystalline structure) is formed when the crystals are heated up to 129 ºC.

Phase III is a transitional solid obtained from phase I cooling, and some studies have shown that it exhibits some important physical properties, such as ferroelectricity. In this phase the crystal forms layers of potassium and nitrates, possibly sensitive to electrostatic repulsions between the ions.

In the layers of phase III, the NO 3 anions lose a little of their planarity (the triangle curves slightly) to allow this arrangement, which, in the event of any mechanical disturbance, becomes the structure of phase II.

Potassium Nitrate Uses / Applications

Salt is of great importance, since it is used in many human activities, which are manifested in industry, agriculture, food, etc. These uses include the following:

  • The preservation of food, especially meat. Despite the suspicion that it is involved in the formation of nitrosamine (a carcinogenic agent), it is still used in delicatessens.
  • Fertilizer, because potassium nitrate provides two of the three macronutrients in plants: nitrogen and potassium. Along with phosphorus, this element is necessary for the development of plants. In other words, it is an important and manageable reserve of these nutrients.
  • It accelerates the combustion, being able to produce explosions if the combustible material is extensive or if it is finely divided (greater surface area, greater reactivity). In addition, it is one of the main components of gunpowder.
  • Facilitates removal of stumps from felled trees. Nitrate supplies the nitrogen necessary for fungi to destroy stump wood.
  • It intervenes in the reduction of dental sensitivity through its incorporation in toothpastes, which increases the protection of the painful sensations of the tooth produced by cold, heat, acid, sweets or contact.
  • It is involved as a hypotensive in the regulation of blood pressure in humans. This effect would be given or interrelated with a change in sodium excretion. The recommended dose in treatment is 40-80 mEq / day of potassium. In this regard, it is pointed out that potassium nitrate would have a diuretic action.

Synthesis

Most of the nitrate is produced in the mines of the deserts in Chile. It can be synthesized through various reactions:

NH 4 NO 3 (aq) + KOH (aq) => NH 3 (aq) + KNO 3 (aq) + H 2 O (l)

Potassium nitrate is also produced by neutralizing nitric acid with potassium hydroxide in a highly exothermic reaction.

KOH (aq) + HNO 3 (conc) => KNO 3 (aq) + H 2 O (l)

On an industrial scale, potassium nitrate is produced by a double displacement reaction.

NaNO 3 (aq) + KCl (aq) => NaCl (aq) + KNO 3 (aq)

The main source of KCl is from the mineral silvin, and not from other minerals such as carnalite or cainite, which are also composed of ionic magnesium.

Physical and chemical properties

Potassium nitrate in solid state appears as a white powder or in the form of crystals with an orthorhombic structure at room temperature, and trigonal at 129 ºC. It has a molecular weight of 101.1032 g / mol, is odorless, and has a pungent salty taste.

It is a compound that is very soluble in water (316-320 g / liter of water, at 20ºC), due to its ionic nature and the ease with which water molecules solvate the K + ion .

Its density is 2.1 g / cm 3 at 25 ºC. This means that it is approximately twice as dense as water.

Its melting points (334 ºC) and boiling points (400 ºC) are indicative of the ionic bonds between K + and NO 3 . However, they are low compared to those of other salts, because the crystalline lattice energy is lower for monovalent ions (that is, with ± 1 charges), and they also have not very similar sizes.

It decomposes at a temperature close to the boiling point (400 ºC) to produce potassium nitrite and molecular oxygen:

KNO 3 (s) => KNO 2 (s) + O 2 (g)

References

  1. Pubchem. (2018). Potassium Nitrate. Recovered from pubchem.ncbi.nlm.nik.gov
  2. Acta Cryst. (2009). Growth and single-crystal refinement of phase-III potassium nitrate, KNO 3 . B65, 659-663.
  3. Marni Wolfe. (October 03, 2017). Potassium Nitrate Risks. Recovered from livestrong.com
  4. Amethyst Galleries, Inc. (1995-2014). The mineral niter. Recovered from galleries.com

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